Process of and compositions for producing coated aluminum and aluminum alloys



PROCESS OF AND COMPOSITIONS FOR PROlJUC- ING COATED ALUlVHNUM AND ALUMINUlW ALLOYS Bruno R. Jeremias, Chicago, lll.,'assignor to Poor & Company, Chicago, 111., acorporation of Delaware N Drawing. Application August 27,1954

. Serial N0. 452,700

14 Claims. c1.

This invention relates to a process of and compositions for produc1ng coated aluminum and aluminum alloys in which the coating is colorless to a distinctive blue to bluish green color, and provides excellent'corrosion re- Y sistance as such or a a'paint basej One of the objects of the invention is to new and improved process for forming a coating on aluminum and aluminum alloys which is highly resistant to corrosion.

Another object is to provide a new and improved. process for forming a distinctive blue to bluish green coating on aluminum and aluminum alloys.

Still'a further object is to providea new and improvedprocess for applying a coating to .aluminum and aluminum alloys whichuadheres to such alloys and also pro-.

vides an excellent paint base to which paints and other organic finishes may be applied.

Another object of theinvention is to provide a process for forming such a coating on aluminum and aluminum alloys in a relatively simple and inexpensive manner.

Still a further object is to produce new and improved 1 coating compositions which, when applied to aluminum or aluminum alloys, form .a film or coating on thesur face thereof, providing excellent corrosion protection.

An additional object of the invention is to provide new and improved corrosion resistant articles made from aluminum and aluminum alloys. Other objects will appear hereinafter.

These objects are accomplished in accordance with the invention by applying to aluminum and aluminum alloys an aqueous solution. consisting essentially of.

chromic acid, phosphoric acid, a complex salt of acetic acid containing combined but undissociated acetic acid,

and fluoboric acid, dissolved in water, with or'without a chloride of a metal (including also ammonium) which dissolves in the solution and does not form insoluble precipitates with the other. ingredients of the solution. The coating solution which is applied to the aluminum or aluminum alloy is heated preferably until a blue to blue-green color develops. to immerse the surface of the aluminum or aluminum alloy in a bath containing the solution which is heated to the. desired temperature. The time of heating'will depend .upon the temperature used and the amount of protection desired. A period ofheating of one minute at a'temperature of. about 120 F. provides substantial corrosion protection but normally produces no substantial color formation on the coated article. A period of fifteen minutes of heating at a temperature of 140 F. is generally sufiicient to develop .a blue-green color. At higher temperatures of, .say 180?-190 F., the bluegreen color can be developed in a shorter period of'time, say, three to five minutes. At temperatures as low as 80 F. no substantial protective coating is provided.

After the aluminum or aluminumv alloy article has been coated the presence of the protective coating can be determined by boiling in water for fifteen minutes., If there is no protective coating, the article'will normally provide a The preferred procedure is.

ice

2 V turn dark. The blue to blue-green coatings, on the other hand, are not substantially affected by the boiling tests.

In the practice of the invention, it is Preferable to; prepare the chromic acid in one solution and the fiuoboric acid in a separate solution. The phosphoric acid and the complex salt of acetic acid containing combined but undissociated acetic acid is preferably included in the first solution. The soluble chloride, if one is used, is preferably included in the second solution. Examples An alternativefiuoboric acid solution has the follow ing composition: i

The above-described solutions may be considered as concentrates suitable for use in preparing'the solutions that are actually employed in coating the aluminum or aluminum alloys; Compositions A and B (or B) are separately preparedbecause if they are mixed together in their concentrated form precipitation of an insoluble substance of unknown composition occurs.

The final composition for coating the aluminum or aluminum alloy is prepared by adding composition A' and composition B to a large volume of water. 'Forexample, a preferred final composition consists of approximately 2% by volume of composition A and 2% by volume of composition B dissolvedin water and a preferred practice'is to immerse the aluminum or aluminum alloy surfaced article in this solution at 150 Fafor 5 to 20 minutes.

The preferred coating solution contains 4 grams 0 CrO 6.3 grams of H PO by weight), 2 grams of sodium diacetate, and 2.68 grams of HBF 'expressed as F (fluorine), dissolved in sufficient water to form a literof solution. The optimum weight ratio of F to CrO is 0.7. The range of F to CrO in the coating solution can vary from 0.44 to about 2.2.

The pH of. the coating solution is preferably within the range of '1 to 2, and the preferred pH range i from 1.15 to1.55. A pH as low as .88 is unsatisfactory.

' The invention will be illustrated but'i' not limited 'by the following examples in which the'quantities are stated in parts by weight unless otherwise indicated. 3

Example 1 position B in water; The weight ratio of F to CrO;

in' this composition 'is approximately .55. The weight ratio of phosphorus as P0 to F in this composition is j approximately 2.08. The weight ratio of 'CrO to about 1.82.

7 Patented Nov. 4, 1958 I A coating -'solution"is prepared by dissolving 1% by volume of composition A and 4% by volume "of composition'B' in water. The weight ratio of F to C10,; in this composition is about 2.2.

Example 4 A coating solution is prepared by dissolving 1% by volume composition A and 2% by volume of composition B in water. The weight ratio of F to CrO in 'this composition is about 1.1. In this "coating 'solu-" tion the weight ratio of phosphorus as P0 to 'F {approximately 1.04. The weight ratio of Cr'O; ito is approximately 0.9. e

Example 5 .A coating solution is prepared by dissolving 1 by volume of composition A and 1% by volume of' composition 'B in water. The weight ratio of 'F 'to 'CrO in 'this composition is approximately .7. The 'weight ratio of CrO to F is approximately 1.43.

Example '6 E amp 7 ;A coating solution is prepared by dissolving 5% -by volume of composition A and 4% by volume of composition :B in water. The weight ratio of F -to Cr-O is approximately .55. The weight ratioof-CrO; to F is approximately 1.78.

Example 8 A coating solution is prepared by dissolving 2% by volume of composition A and 2% by'volume'ofcomposition 3 in water. tion is approximately 1.5.

Example 9 Each of the solutions describedrin Examples .1 to 8 is applied as a coating to wrought aluminum, alloys :28, 38, 248, 618 and 758, respectively, by immersing a plate or otherarticle formed from such alloys in each of the coating solutions for periods of time from 1 to "20 minutes at temperatures of from 1-20" to 150 F.

At a temperature of 120 F. in an immersion :time .of 1 minute in a coating solution of Example 8, -abright transparent colorless corrosion-resistant coating was obtained..

-In the coating solution of Example 8 :an excellent blue to blueegreen color is developed at a temperature of 150 F. in an immersion time from '5 to 20 minutes. In general, the higher'the temperature and the longer the time 'of immersion, the deeper the color. While it is possible with some alloys to carry out the process at temperatures up to the boilingpoint of the solution, with other alloys, such as 24S,'itis desirable to operate at temperatu1=es around 150 vF. 7

E am 11 c d na am um nd a uminu al o in about minutes.

The pH of the resultantcomposi- Example 11 In the coating solution of Example 3 a deeper color aluminum and aluminum alloys at ll90 F. in one,

minute. e "In practicing the invention, 'the chromium contained in the coating solution is used up during the pro'c'essfand V the fluoboric acid is removed to some extent from the a coating solution by dragout. The chromium content is maintained by adding additional quantities of composition A, andthe'fluorbori'c acid content is maintained by adding additional quantities of composition B t or composition B. The pH is also maintained by the addition of compositions A and B (or B). 7

After the aluminum and aluminum alloy articles are removed from the coating'soluti on they are 'rinsedvtlith water which rnay be cold 21" is preferably not heated to; temperatures higher than F.

'Generally'spealing, the optimum operating conditjons involve the use of a temperature of the coating solution 7 of F., a pH of about 135 and animmersion time] of 10 to '20 minutes or longer, if necessary, until the desired "color is obtained, but ordinarily the immersion time should not exceed 3.0 minutes because the "coating is'not'adherjent. Even B'OminutesiS too long with some alloys. V The complexsalts of acetic acid containing combined I but undissociated acetic acid are derived by replacing oneor morernolecules of water in sodium acetate "w th; amdlecule o'f undissocate'd'acetic acid and are well known chemicals. Examples of such substances 'are the calcium, sodium and potassium salts. However, sodium Qdiace't'at is the principal salt of this type available commercially,

The function'of the complex water soluble salt o facetic,

acid containing combined but undissociated acetic acid is not clearly understood 'but it serves in some way"to l permit the aluminum or aluminum alloy to be immersed? in the chromic'acid solution for a longer period'of tiinethan would be otherwise possible and still obtain a satisfactory product. This in turn makes it possible to. obtain the optimumtdeposition of chromium and a deep blue color without destroying the adherence of the chromium film to the aluminum or aluminum alloy.'I

The quantity of complex acetate in the coating solution, 7 calculated as'sodium diacetate, is preferably 0.1 to 5.01. grams per liter. Larger amounts which do-not cause. l

discoloration can be employed but serve no useful put-, pose.

The employment of the ammonium chloride .or other chloride, such as sodium-chloride or potassiumch loride, is optional and the quantity used'may be varied. In general, however, the quantity of ammonium chloride 1 based on the total weight of HBF and NH cl is with. inthe range of 4% to 32% by weight. The employ;

ment of the chloride apparently results in the for-ma. tion of a stronger 'film 'on the aluminum or alumin um. alloy. V

'The weight ratio of phosphorus at PO; to F int he V coating "solution is preferably within the range of about. 1:2 to 4.5:1.

The invention provides a .new and improved ,process organic finishes.

'The .term aluminum alloy asused hereinsrefers to.

alloys of aluminum and othenmetals .in'whichamrninn a m vthe "majo Pr n y :weigh of t a low Such alloys are well known. in the art and include casting alloys as well as. wrought alloys. Wrought aluminum alloy 28 containsa minimum of 99% by Weight of aluminum. Alloy 38 contains approximately 1.2% by weight manganese, the remainder being aluminum. Alloy 24S contains 4.5% by weight copper, 0.6% by weight manganese, 1.5 by weight magnesium, and the remainder aluminum. Alloy 618 contains 0.6% by weight silicon, 0.25% by weight copper, 1.0% by weight magnesium and 0.25% by weight chromium, the remainder being aluminum. Alloy 758 contains 1.6% by weight copper, 2.5% by weight magnesium, 0.3% by weight chromium and 5.6% by weight zinc, the remainder being aluminum. Of the casting alloys, casting alloy No. 13 contains about 12.0% by weight silicon, the remainder being aluminum. Casting alloy No. 43 contains 5.0% by weight silicon, the remainder being aluminum. Casting alloy No. 85 contains 4.0% by weight copper and 5.0% by weight silicon, the remainder being aluminum.

The expressions aluminum surfaced article and aluminum alloy surfaced article are intended to include and cover articles in which at least the surface of the article is composed of aluminum and aluminum alloys respectively.

The invention is hereby claimed as follows:

1. A process of applying a protective coating to aluminum and aluminum alloys which comprises applying to aluminum and aluminum alloys with heating to a temperature above about 120 F. an aqueous solution consisting essentially of chromic acid, phosphoric acid, a complex water soluble salt of acetic acidcontaining combined but undissociated acetic acid derived by replacing water in an acetate salt with acetic acid, and fluoboric acid dissolved in water, the weight ratio of F to CrO in the coating solution being within the range from 0.44 to about 2.2, the quantity of said complex salt being at least 0.1 gram per liter of solution,

and the pH of the coating solution being within the range from about 1 to about 2 until a protective coating is formed.

2. A process of applying a protective coating to aluminum and aluminum alloys which comprises applying to aluminum and aluminum alloys with heating to a temperature above about 120? F. an aqueous solution consisting essentially of chromic acid, phosphoric acid, a complex water soluble salt of acetic acid containing combined but undissociated acetic acid derived by replacing water in an acetate salt with acetic acid, and fluoboric acid dissolved in water, the weight ratio of F to CrO in the coating solution being approximately 0.7, the quantity of said complex salt being at least 0.1 gram per liter of solution, and the pH of the coating solution being within the range from about 1.15 to about 1.55 until a protective coating is formed.

3. A process of applying a protective coating to aluminum and aluminum alloys which comprises applying to aluminum and aluminum alloys with heating to a temperature above about 120 F. an aqueous solution consisting essentially of chromic acid, phosphoric acid, a complex water soluble salt of acetic acid containing combined but undissociated acetic acid derived by replacing water in an acetate salt with acetic acid, and fluoboric acid dissolved in Water, the weight ratio of F to CrO in the coating solution being within the range from 0.44 to about 2.2, the quantity of said complex salt being at least 0.1 gram per liter of solution, and the pH of the coating solution being within the range from about 1 to about 2 until a blue to blue-green coating is formed.

4. A process of producing an aluminum alloy containing a blue to blue-green coating which is adherent and resistant to corrosion comprising immersing the surface of the aluminum alloy in an aqueous acetic solution consisting essentially of chromic acid, phosphoric 6 acid, sodium diacetate and fluoboric. ac'id dissolved 'in water, the weight ratio of to CrO in the resultant so-' lution being within the range from 0.44 to about 2.2, the quantity of sodium diacetate being within the range of 0.1 to 5 grams per liter of solution, the pH of said solution being Within the range from 1.15to 1.55, the temperature .of said solution being within the range from F. to about for a period of time from 5 to 20 minutes.

5. A process as claimed in claim 4 in which said solution contains 4% to 32% by weight, calculated as NH CI, of a water soluble ionizable chloride which does not form insoluble precipitates with the other ingredients of the solution.

6. A process as claimed in claim 5 in which said chloride is ammonium chloride.

7. A coating composition for aluminum and aluminum alloys consisting essentially of chromic acid, phosphorus. acid, a complex water soluble salt of acetic acid containing combined but undissociated acetic acid derived by replacing water in an acetate salt with acetic least 0.1 gram per liter of solution, and the pH being within the range from 1 to 2.

8. A coating composition for aluminum and aluminum alloys consisting essentially of chromic acid, phosphoric acid, a complex water soluble salt of acetic acid containing combined but undissociated acetic acid derived by replacing Water in an acetate salt with acetic acid, and fluoboric acid dissolved in water, the weight ratio of F to CrO beingabout 0.7, the quantity of said complex salt being at least 0.1 gram per liter of solution, and the pH being within the range from 1.15 to 1.55.

9. A coating composition for aluminum and aluminum alloys consisting essentially of chromic acid, phosphoric acid, sodium diacetate and fluoboric acid dissolved in' 10. A coating composition for; aluminum and alumi-- num alloys consisting essentially of chromic acid, phosphoric acid, sodium diacetate and fluoboric acid dissolved in water, the weight ratio of F to Cr0 being within the range from 0.44 to about 2.2, the quantity of sodium diacetate being within the range of 0.1 to- 5 grams per liter of solution, and the pH being within the range from 1.15 to 1.55.

11. A coating composition consisting essentially of chromic acid, phosphoric acid, sodium diacetate and fluoboric acid dissolved in Water in the approximate pro portions of 4 grams of CrO 6.3 grams of 75% by weight aqueous H PO 2 grams of sodium diacetate, and 2.68. grams of HBF calculated as F, per liter of solution. I

12. A coating composition for aluminum and aluminum alloys consisting essentially of chromic acid, phosphoric acid, sodium diacetate, fluoboric acid and ammonium chloride dissolved in water, the weight ratio of F to CrO being within the range from 0.44 to about 2.2, the pH being within the range from 1.15 to 1.55,

the weight ratio of phosphorus as P0,, to F being within V 13. Aluminum and aluminum alloys containing a blue to blue green protective coating produced by the process of claim 1.

Coates Jan. '18, 1:949"

Bruno 61: 1 May 16,1959

Sprnance c't a1. ..IMay 1 1,:1954

FOREIGN --PATEN51"S 

7. A COATING COMPOSITION FOR ALUMINUM AND ALUMINUM ALLOYS CONSISTING ESSENTIALLY OF CHROMIC ACID, PHOSPHORUS ACID, A COMPLEX WATER SOLUBLE SALT OF ACETIC ACID CONTAINING COMBINED BUT UNDISSOCIATED ACETIC ACID DERIVED BY REPLACING WATER IN AN ACETATE SALT WITH ACETIC ACID, AND FLUOBORIC ACID DISSOLVED IN WATER, THE WEIGHT RATIO OF F TO CRO3 BEING WITHIN THE RANGE FROM 0.44 TO ABOUT 2.2, THE QUANTITY OF SAID COMPLEX SALT BEING AT LEAST 0.1 GRAM PER LITHER OF SOLUTION, AND THE PH BEING WITHIN THE RANGE FROM 1 TO
 2. 